I use an iPhone 6S on Visible. Due to its age, the battery isn't doing so great. Because of this, I have a tendency to put the phone on low power usage mode. You do this by going into settings, Cellular, Cellular Data Options, toggle the Low Data Mode on here. Also, under settings, Battery, toggle the Low Power Mode here as well.
I mention this because It seems the iPhone has a bad tendency to use cell bandwidth for whatever and whenever it wants to use it on. The low power modes prevent this.
I consistently get better speeds using low power mode vs normal. Something for iPhone owners to try. Please report back and tell me if you see a difference as I do.